Thick film dielectric electroluminescent (TDEL) devices for displays and other applications are described, for example, in U.S. Pat. Nos. 5,432,015 and 6,919,126 (the disclosures of which are incorporated herein in their entirety). In comparison to conventional thin film electroluminescent (TFEL) devices, TDEL devices provide superior resistance to dielectric breakdown, reduced operating voltage and substantially increased luminance.
In general TDEL devices have a thick film dielectric composite structure deposited on a glass, glass ceramic or ceramic substrate that withstands processing temperatures sufficiently high to facilitate deposition and annealing of phosphor films that can provide high luminosity. The thick film dielectric layers used in these displays have a high dielectric constant, allowing the use of relatively thick dielectric layers in the displays without a significant increase in the display operating voltage. The relatively thick dielectric layer, typically greater than 10 μm, is used to prevent dielectric breakdown during display operation. Typically, the thick film dielectric composite structure comprises a layer of a sintered perovskite piezoelectric or ferroelectric material such as lead magnesium niobate (PMN) or lead magnesium titanate-zirconate (PMN-PT) with a dielectric constant of several thousand. To form the composite there is typically applied a thinner overlayer of a compatible piezoelectric or ferroelectric material such as lead zirconate titanate (PZT) using metal organic deposition (MOD) or sol gel techniques to smooth the thick film surface for deposition of a thin film phosphor structure.
U.S. Pat. No. 7,112,373 describes a TDEL device having a europium activated barium thioaluminate phosphor film that emits blue light with high luminosity. U.S. Patent Application Serial No. 2004/0135495 describes a full colour display having a thick film dielectric composite structure and an europium activated barium thioaluminate phosphor film to generate blue light and having blue, green and red sub-pixels wherein the green and red sub-pixels have overlying photoluminescent films that convert the blue light to green and red light respectively. In this structure the blue light is completely absorbed in the green and red photoluminescent films to provide green and red light to meet the CIE colour coordinate requirements for full colour video displays. The green and red photoluminescent films for this display consist of pigment particles formulated using organic dyes dispersed in an optically transparent polymer film. However, the photoluminescence efficiencies of these films degrade differentially with operating time due to exposure to the blue light used for excitation and to ultraviolet light from the ambient environment. This differential degradation causes colour shifts with increasing operating time.
White light sources consisting of blue light-emitting diodes (LEDs) with a coating comprising inorganic photoluminescent materials to convert some of the blue light to longer wavelength light are known. Yttrium aluminum garnet (YAG) has been used as the photoluminescent colour conversion layer in combination with these LEDs to generate white light. U.S. Pat. No. 5,998,925 discloses the use of a cerium doped YAG phosphor in combination with a blue light emitting LED to provide a white light source. The YAG-based materials used with these devices provide for a stable light output, however, the light sources are essentially point sources due to the small size of the primary blue light-generating LEDs making them unsuitable for applications requiring an extended light source with uniform luminosity. U.S. Pat. No. 7,362,048 teaches how the YAG materials can be doped with additional elements such as gadolinium to shift the colour temperature of the white light to a desired value. U.S. Pat. No. 7,417,368 discloses a TDEL pixel array having a europium activated barium thioaluminate phosphor film that generates blue light that is absorbed by a cerium doped YAG as a photoluminescent overlayer to generate yellow light. U.S. Pat. No. 7,391,060 discloses a TDEL device having a barium thioaluminate blue-emitting phosphor layer and a photoluminescent layer that converts blue light to red or green light.
It is therefore desirable to provide an extended area uniform light source providing uniform luminance for displays and other applications as well as displays and applications incorporating the light source and methods of making therefor. The uniform light source is achieved using a combination of a blue light-emitting electroluminescent layer and a highly stable inorganic photoluminescent colour conversion layer to tailor the colour spectrum of the emitted light and provide a uniform white illumination.